The present invention relates to a method and an apparatus for plotting a trace pattern on the tread band of a tire especially when making a prototype tire for subsequent testing.
In greater detail, the method and apparatus of the present invention are designed to execute the plotting of a trace pattern on vulcanized tires the tread band of which, first smooth, must be submitted to a so-called "gouging" working adapted to produce grooves and cuts suitably oriented according to a predetermined pattern.
It is known that a gouging operation, generally carried out on small sampling tires intended for control tests or other particular uses, comprises manually forming by a tool essentially made of a suitably shaped and heated blade, a plurality of cuts and grooves having appropriate width, depth and orientation features, so as to form grooves or furrows in the tread band itself according to a specific pattern, usually referred to as "tread pattern".
In order that an operator may carry out cuts and grooves on the tread band according to the exact pattern as previously defined, it is necessary that plotting of the path to be then followed be first executed on the tread band.
The assembly of the marks relating to cuts and grooves substantially defines a two-dimension pattern on the tread surface which is usually referred to as "trace pattern" in order to distinguish it from the three-dimension tread pattern generated by the actual presence of said cuts and grooves (imprint or track pattern).
Presently, plotting of the trace pattern on the tread band is usually carried out by a painting operation executed on an inflated tire through appropriate masks of a metal sheet cut according to the pattern to be reproduced. Generally, a mask used for plotting a trace pattern is so sized that it covers the whole tread band axial width and the sidewall portions that may extend as far as the maximum carcass width of the tire along a circumferential portion equal to a submultiple integer of the overall circumferential extension.
The pattern plotting is thus carried out on consecutive circumferential sectors and for the execution of same, therefore, the mask must be each time positioned exactly in coincidence with the end portion of the previously plotted sector. In carrying out this positioning operation the greatest care must be taken, in that possible inaccuracies could lead to the impossibility of making the end of the last sector to be plotted fit the start portion of the first sector previously plotted.
Often a perfect matching of said ends is impaired in that unavoidably the circumferential extension of the tread band is indeterminate and imprecise as the tolerance field of said extension generally involves a few millimeter deviation from the nominal value.
In addition it will be recognized that in carrying out cut workings in the metal sheet for preparing the mask it is to be taken into account the fact that the geometrical features of the pattern formed therein will be subjected to modifications when the mask itself, first extended in a flat plane, must be adapted to the curved conformation of the tread band, which, as known, is bent in two directions at right angles to each other, that is, the axial and circumferential directions.
In other words, the cutting operation of the mask must be executed according to a flat extension of the trace pattern to be plotted on the tread band.
In view of the above, making cuts adjacent to the side edges of the tread band is particularly critical. In fact, at said areas important deformations are likely because of the necessity to bend the metal sheet forming the mask according to a substantially radial orientation on the tire sidewall. In order to meet this requirement, the side portions of the metal sheet are divided into a plurality of ribbon-like portions disposed consecutively in side by side relation, which portions will be partly overlapped when the mask is fitted to the tire conformation.
In addition said slits in the mask cannot be made such as to exactly mate the trace pattern to be reproduced. In particular, it is impossible to make slit combinations extending according to an endless line from one end of the pattern to the other, in that this would cause the mask to be divided into a great number of small pieces. It is therefore necessary that the slit extension be suitably interrupted for enabling the different mask portions defined by the slits themselves to be kept conveniently linked to each other. Therefore the interruption stretches of the slits arranged on purpose must be conveniently identified in the trace pattern produced on the tread band, so that the operator designated to carry out gouging may be able to distinguish them from other possible interruptions that on the contrary must be really reproduced to carry out the tread pattern.
Attempts have been made to overcome said difficulties by directly furrowing the vulcanized tire surface without the aid of guide masks: for example, in Canadian Patent No. 1,157,103 a laser beam is used for forming a plurality of slits and grooves on the outer tire surface, on the tread or sidewalls, which are adapted to define a tread pattern or identification elements for the tire, respectively.
In order to achieve the above objective, the tire, rotating about its own axis, is mounted to a support moving according to three Cartesian axes: the groove and cut shape and depth are obtained by controlling the support movements so as to adjust the fixed position of the focal point of the laser to the surface portion of the tire to be worked.
Such a solution however has not solved the problem in a satisfactory manner; the relative movement between the laser and tire surface only extends along said Cartesian axes and about the rotational axis of the tire so that only substantially flat surfaces can be worked; in other words, only the tire tread band and sidewall can be worked in separate steps and alternately. 0n the contrary, working of the tire shoulder, that is the curved interconnection portion between the sidewall and tread end is either impossible, or very difficult and the results are poor.
In addition, in order to enable execution of the pattern plotting point by point, the position of the tire relative to the laser beam focus must be continuously adjusted by moving the support: since the machine is provided with bulky and solid movement members, the machine is slow and also expensive if a sufficient working precision is to be ensured; in spite of the above, the pattern thus achieved is not comparable from a qualitative point of view to those obtained through gouging and moulding, in particular due to the difficulty of executing sharp edges in the rib-block region, so that the tire is not adapted for executing some specific behavior tests.